Magnetic resonance imaging (MRI) scanners use a magnetic field and pulses of radio wave energy to create image of organs and structures inside of a body. A conventional MRI scanner comprises RF transmitters which are all operated on the basis of amplification of an analog input signal with only one high power RF power amplifier. RF transmitters of conventional MRI scanners are illustrated in FIG. 1. However studies are progressing to make a multi-channel, all-digital RF transmitter for new generation MRI scanners. Via new generation multi-channel MRI RF transmitter with digital modulation method, each transmitter channel can be reconfigured individually. Thereby, many parameters like signal type, frequency of operation, phase and amplitude information for RF shimming can be changed easily from a control computer. High image quality with RF shimming capability is expected to be achieved with new generation MRI multi-channel RF transmitters. For this purpose, the RF transmitters of the new generation MRI scanners must be multi-channel, able to generate and transmit RF signals digitally, use digital RF power amplifiers (Switch Mode Power Amplifiers). An exemplified new generation multi-channel RF transmitter block diagram is illustrated in FIG. 2.
Due to its high efficiency performance, one of the preferred digital RF transmitter architecture for new generation MRI scanners is Class-S RF transmitters. In a conventional Class-S RF transmitter, the input signal is converted into a digital signal with constant amplitude by DSM or any other Analog Digital Converter (ADC) and the digital signal is amplified by a switch mode power amplifier. The amplitude information of analog Sinc signal varying in amplitude is transmitted into pulse width in time domain by DSM. Thus a digital signal, pulse width of which is representing the amplitude information of the analog signal, without varying amplitude is achieved. Afterwards, the analog signal is converted into digital modulated signal at RF carrier frequency by multiplying the RF carrier clock signal. After, the digital modulated signal at the carrier frequency is amplified by a power amplifier and converted into analog signal by a band pass filter. So, an analog amplified signal at desired RF frequency is achieved and the said signal can be radiated by MRI antenna in order for medical imaging. An exemplified Class-S RF transmitter block diagram is illustrated in FIG. 2. However, the Class-S RF transmitters have a critical disadvantage that to achieve high linearity there is a necessity to keep the Delta Sigma Modulation (DSM) sampling frequency signal applied to the input of the Class-S RF transmitters high. By this way, the quality of the signal applied to the input is maintained at the output of the Class-S RF transmitter. Nevertheless, switching the Class-D RF power amplifier, inside the Class-S RF transmitter architecture, at high frequency not only decreases the efficiency but also necessitates the RF power amplifier to be wide band, because the more the switching frequency, the wider the frequency band. Chaotically, using wide band architecture for Class-D RF power amplifier leads to decrease the efficiency (total signal power to desired signal power ratio is high) and signal linearity. On the other hand, as aforementioned, low sampling frequency of the DSM signal applied to the input of the Class-D RF power amplifier leads to a corruption in the DSM signal frequency at the output of the Class-D RF power amplifier.
One of the main parameters that affect the performance of MRI scanners is the signal quality. The above described transmitters use only one bit signal having a low SNR rather than multibit signals. Moreover, the DSM used in the conventional Class-S RF transmitters (FIG. 2) has the best efficiency for constant amplitude (non-varying) signal. For said signal, low amplitude values according to peak value in the input signal are poorly sampled. Therefore, while the peak to average ratio value of the signal increases, the SNR decreases and total signal power to desired signal power ratio value in the DSM signal spectrum increases. To increase the SNR, multibit DSM structures can be used but with the said structures constant amplitude signal cannot be generated at the output. Since digital amplification cannot be applied to non-constant amplitude signal, the efficiency decreases. As a result of the above mentioned deficiencies, the efficiency advantage of the Class-S RF transmitters cannot be used for MRI scanners.
In the state of the art EP2755323A1 publication discloses an RF signal generating circuit that generates, from a digital signal, an RF pulse signal to be radio-transmitted.
In the state of the art Ville Saari et al. “13.5 MHz Class-S Modulator for an EER Transmitter” NORCHIP CONFERENCE, 2004 publication discloses an integrated 13.SMHz class-S modulator for an EER transmitter. The modulator uses 3.3 V supply voltage and was fabricated using 0.18 pm CMOS technology.
In the present invention, peak to average amplitude ratio of the input signal is decreased. Thereby, the input signal can be sampled efficiently at all amplitude values.